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Carbonic anhydrase and cardiac pH regulation
J I Vandenberg1, N D Carter, H W Bethell
1Department of Biochemistry, University of Cambridge, United Kingdom.
Insights
Membrane-bound carbonic anhydrase (CA) in ferret hearts aids intracellular pH recovery after ischemia. Inhibiting CA slows pH changes, highlighting its crucial role in cardiac tissue.
Area of Science:
- Cardiovascular Physiology
- Biochemistry
- Cellular Biology
Background:
- Membrane-bound carbonic anhydrase (CA) has been identified in mammalian cardiac tissue.
- Understanding the specific location and function of CA in the heart is crucial for cardiac health research.
Purpose of the Study:
- To investigate the histochemical location of CA in the ferret heart.
- To determine the functional role of CA in regulating intracellular pH (pHi) and recovery from ischemia.
Main Methods:
- Histochemical staining (modified Hansson's technique) to locate CA.
- Langendorff-perfused ferret heart model to assess pHi changes using 31P nuclear magnetic resonance spectroscopy.
- Application of membrane-permeable (6-ethoxzolamide) and impermeable (CL-11,366) CA inhibitors.
Main Results:
- CA was localized to capillary endothelial and sarcolemmal membranes in ferret hearts.
- CO2 washout caused a transient rise in pHi, which was significantly reduced by CA inhibitors.
- CA inhibitors slowed the rate of pHi recovery and contractile function after myocardial reperfusion.
Conclusions:
- Carbonic anhydrase facilitates CO2-H2CO3 hydration-dehydration, influencing intracellular and extracellular CO2 concentrations.
- CA enhances CO2 transfer from intracellular to extracellular compartments, significantly aiding pHi recovery post-ischemia.
- Targeting CA may offer therapeutic potential for managing cardiac ischemia and reperfusion injury.
Abstract:
Membrane-bound carbonic anhydrase (CA) has recently been identified in mammalian cardiac tissue. In this study, we have investigated the histochemical location and functional role of CA in the ferret heart. Heart sections stained by a modified Hansson's technique showed CA to be located on capillary endothelial membranes as well as on sarcolemmal membranes. In the Langendorff-perfused heart, washout of CO2 brought about by switching perfusion between 25 mM HCO3(-)-5% CO2-buffered solution and nominally HCO3(-)-CO2-free solution caused a transient rise in intracellular pH (pHi) measured by the chemical shift of 2-deoxy-D-glucose 6-phosphate with 31P nuclear magnetic resonance spectroscopy. The initial rate of change of pHi, measured over the first 60-75 s of CO2 efflux, was significantly reduced from 0.41 +/- 0.03 pH units/min (n = 9) in control hearts to 0.28 +/- 0.02 pH units/min (n = 5) in the presence of the membrane-permeable CA inhibitor 6-ethoxzolamide (P < 0.05 compared with control) and to 0.22 +/- 0.04 pH units/min (n = 5) in the presence of the membrane-impermeable CA inhibitor CL-11,366 (P < 0.01 compared with control). After reperfusion of the ischemic myocardium, both CA inhibitors caused a significant slowing of initial rate of change in pH (and initial rate of recovery of contractile function) compared with control hearts. These results suggest that CA, by facilitating the hydration-dehydration of CO2-H2CO3, alters the relative concentrations of CO2 inside and outside the cells, thus enhancing the rate of CO2 transfer from the intracellular to extracellular compartments, which contributes significantly to pHi recovery after reperfusion of the ischemic myocardium.